1. Field of the Invention
The present invention relates generally to a cell search apparatus and method in a mobile communication system, and in particular, to an apparatus and method for performing a multisearch operation on a plurality of cells to be searched.
2. Description of the Related Art
In general, a mobile communication system is classified into a synchronous system developed and used in the United States and an asynchronous system developed and used in Europe. With the rapid development of the mobile communication industry, there have been demands for a future mobile communication system which can provide not only a voice service but also a multimedia service including data and image services. 3rd Generation Partnership Project (3GPP) has been standardizing both the synchronous future mobile communication system and the asynchronous future mobile communication system. The synchronous future mobile communication system is called a “code division multiple access-2000 (CDMA-2000) system,” and the asynchronous future mobile communication system is called a “wideband-CDMA (W-CDMA) system.” The W-CDMA mobile communication system comprises UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Networks (UTRANs) that perform an asynchronous operation between Node Bs. Therefore, in order to identify the Node Bs, unique scrambling codes are assigned to the Node Bs. For example, if the number of cells, or Node Bs, constituting the UTRAN is 512, each of the 512 Node Bs is assigned its own unique scrambling code among 512 scrambling codes, for its identification. Since a Node B transmits a signal with a scrambling code for its identification, a user equipment (UE) must be able to identify a scrambling code for the Node B in order to normally receive a signal provided from the Node B.
That is, in order to normally receive a signal provided from a Node B, a UE must determine a scrambling code of a signal received at the highest energy among signals received from its neighboring Node Bs, and such a process of determining a scrambling code is called “cell search.” The cell search is performed in various ways according to circumstances, and includes (i) initial cell search for acquiring pseudo noise (PN) code timing by a UE upon power on, (ii) multipath search for detecting a multipath signal component of a received signal for rake demodulation while continuously maintaining the acquired PN code timing, (iii) neighbor cell search for searching target neighbor cells to which the UE is to be handed off when the UE is located in a handoff region, and (iv) re-acquisition for re-acquiring PN code timing, which was lost when the UE selects a slotted mode in an idle state or wakes up from a sleep state.
In the W-CDMA mobile communication system, a UE, as stated above, must perform cell search by measuring a phase of each of all scrambling codes that are assignable to Node Bs, i.e., 512 scrambling codes, in order to determine a scrambling code for a Node B to which the UE itself belongs (hereinafter, referred to as a “source Node B”). However, such a cell search algorithm for determining phases of all scrambling codes for the Node Bs one by one requires a long cell search time. To solve this problem, a new multistep cell search algorithm has been proposed.
The multistep cell search algorithm will now be described below.
Node Bs constituting the W-CDMA mobile communication system are assigned their own unique cell identification codes, or scrambling codes, for identification of the Node Bs. A UE identifies the Node Bs with the assigned scrambling codes. It will be assumed herein that the number of cells constituting the W-CDMA mobile communication system is 512 and one cell exists in each Node B, so the number of Node Bs constituting the W-CDMA mobile communication system becomes 512. However, the Node B may employ one or more cells. In this case, the 512 Node Bs are assigned their own unique scrambling codes, and a UE identifies the Node Bs using the unique scrambling codes assigned to the Node Bs.
Currently for a cell search, in order to search a source Node B, a UE must perform a cell search on each of the 512 Node Bs constituting the W-CDMA mobile communication system. Since performing a cell search on each of the 512 Node Bs constituting the W-CDMA mobile communication system is equivalent to measuring a phase of each of all the scrambling codes for the 512 Node Bs, the UE requires a large amount of time to perform the cell search. As a result, it is inefficient to apply the existing cell search algorithm to each of the Node Bs constituting the W-CDMA mobile communication system. Therefore, the UE employs the multistep cell search algorithm. In order to realize the multistep cell search algorithm, a plurality of Node Bs, e.g., 512 Node Bs, belonging to the W-CDMA mobile communication system are divided into a predetermined number of groups, e.g., 64 groups Group#0 to Group#63. The divided 64 Node B groups are assigned their own unique group identification codes, for identification of the Node B groups. In addition, each Node B group includes 8 Node Bs, and the 8 Node Bs are assigned their own unique scrambling codes for spreading a common pilot channel (CPICH), so that the UE can search for its source Node B, using the assigned scrambling codes.
The multistep cell search algorithm includes in the first-step a search, in the second-step a cell search and in the third-step another cell search. In the first-step cell search process, a UE receives primary synchronization channel (P-SCH) signals transmitted from a Node B and acquires slot timing for a signal received at the highest power among the received P-SCH signals. In the second-step cell search process, the UE acquires slot synchronization based on the slot timing information acquired in the first-step cell search process, and then acquires frame synchronization and detects a Node B group to which the UE belongs by receiving a secondary synchronization channel (S-SCH) transmitted from the Node B. In the third-step cell search process, the UE finally searches for its source Node B using a scrambling code for the Node B by receiving a CPICH signal transmitted from the Node B based on the frame synchronization and Node B group information acquired in the second-step cell search process.
The multistep cell search will now be described with reference to FIG. 1.
FIG. 1 illustrates an example of a structure of synchronization channels in a general W-CDMA mobile communication system. Referring to FIG. 1, in the W-CDMA mobile communication system, a searcher in a UE uses a synchronization channel (SCH) and a common pilot channel (CPICH), for synchronization. Node Bs transmit the CPICH using their own unique scrambling codes. A period of the scrambling code is equal to a one-frame length. In the W-CDMA mobile communication system having the channel structure illustrated in FIG. 1, as many Gold codes (not shown) having a period of 218-1 as a one-frame length are used as the unique scrambling codes. Only M (=512) Gold codes among all available Gold codes are used. The SCH, a downlink channel used for cell search by the UE, comprises two subchannels primary SCH (P-SCH) and secondary SCH (S-SCH). Since each time slot of the P-SCH and S-SCH comprises 2,560 chips and 15 time slots constitute one radio frame, one radio frame, therefore, comprises 38,400 chips. Further, as illustrated in FIG. 1, the P-SCH and S-SCH are transmitted by N chips, i.e., 256 chips, amounting to 1/10 of the chips transmitted over one time slot, at the beginning of each time slot. In addition, as orthogonality is maintained between the P-SCH and the S-SCH, it is possible to overlap the two channels during transmission. The P-SCH, a channel over which the 512 Node Bs constituting the W-CDMA mobile communication system transmit the same code, transmits a primary synchronization code (PSC) acp. Further, the P-SCH transmits as much primary synchronization code as 1/10 period, i.e., 256 chips, of a time slot at each time slot. The UE acquires the slot timing from a Node B by receiving a P-SCH signal transmitted from the Node B.
The Node B transmits an S-SCH signal along with a P-SCH signal. If the 512 Node Bs constituting the W-CDMA mobile communication system are divided into 64 Node B groups, the S-SCH transmits a secondary synchronization code (SSC) acsi,k, i.e., a Node B group identification code to which the Node B belongs. The secondary synchronization code is a 15-symbol sequence having a 256-chip length. In the secondary synchronization code, acsi,k, i (=0, 1, . . . , 63) represents the number of Node B groups, i.e., the number of scrambling code groups, where k (=0, 1, . . . , 14) represents a time slot number. Further, the secondary synchronization code is selected from 16 code groups having a 256-chip length. A sequence of the secondary synchronization code represents a code group to which a downlink scrambling code for a corresponding Node B belongs. The code group further represents a group of codes for generating the Node B group identification code. The secondary synchronization channel also transmits the secondary synchronization code using 1/10 period of a time slot, i.e., using 256 chips, at each time slot. The UE detects a Node B group which is its source Node B and synchronizes with a frame boundary by receiving a secondary synchronization channel signal transmitted from the Node B.
The Node B group identification code is used to determine a group to which the Node B belongs. A comma-free code is typically used as the Node B group identification code. The comma-free code comprises 64 codewords, and each codeword comprises 15 symbols. The 15 symbols are transmitted every frame. However, as mentioned above, values of the 15 symbols are mapped to one of the secondary synchronization codes acsi,k of acsi,0, acsi,1, . . . , acsi,15 before being transmitted. That is, as illustrated in FIG. 1, an ith secondary synchronization code corresponding to a symbol value i is transmitted every time slot. In addition, 64 codewords of the comma-free code identify 64 code groups. The comma-free code is characterized such that the cyclic shift of each codeword is unique. Therefore, it is possible to acquire information on a code group, i.e., a Node B group, where the UE belongs, and information on frame synchronization by receiving a secondary synchronization channel signal for a period of several time slots, correlating secondary synchronization codes with a secondary channel signal for the period of several time slots, and performing a cyclic shift operation on each of the 64 codewords 15 times. The term “frame synchronization” means synchronization on timing or phase within one period of a scrambling spreading code for a spread spectrum system. In the existing W-CDMA mobile communication system, one period of a spreading code and a length of a frame are both 10 ms, so this is called “frame synchronization.”
After performing the first-step cell search process and the second-step cell search process, the UE can acquire information on slot synchronization, Node B group identification code and frame synchronization through the P-SCH and S-SCH. However, since the UE cannot distinguish a scrambling code for its source Node B from 8 other Node B scrambling codes within a code group based on the acquired Node B group identification code, scrambling code synchronization is not completed achieved yet. Therefore, the UE can identify a scrambling code to be used among the 8 scrambling codes by performing correlation on each of the 8 scrambling codes belonging to the code group for the CPICH via a third-step cell search process.
The UE must periodically measure the strength of the signals transmitted from Node Bs neighboring its source Node B, in order to receive an optimal multipath signal from the Node B in a radio channel environment or a handoff situation. In this case, the UE acquires timing information of neighbor Node Bs by the multistep cell search algorithm or from the current Node B in service, receives CPICH from each of the corresponding Node Bs, and periodically performs correlation on each of the received CPICH signals. Herein, a process of periodically measuring strength of signals from Node Bs neighboring a source Node B to acquire the timing information of the neighboring Node Bs will be referred to as a “multipath search” in order to distinguish this process from the above-stated cell search process that is performed before the UE acquires timing of the neighbor Node Bs.
Generally, the UE identifies signals from Node Bs to which it may probably move, by cell search, and continuously manages the Node B signals through the multipath search. Further, the UE demodulates valid multipath signal components, i.e., valid multipath signals, among the signals received by the cell search and multipath search. In the case of the multipath search, the UE should be able to perform a high-speed search in order to promptly respond to an abrupt variation in channel conditions. The multipath search calculates correlation values for all hypotheses at stated intervals in a window and then detects a plurality of correlation values which are peak values and larger than or equal to a predetermined threshold value. Then, scrambling code timings having the detected correlation values become timings of multipath signal components.
As stated above, the W-CDMA mobile communication system acquires scrambling code synchronization while continuously performing the third-step cell search. That is, a UE acquires time slot synchronization with P-SCH in the first-step cell search process, and acquires frame synchronization and scrambling code group information, i.e., group information of its source Node B, with S-SCH in the second-step cell search process. Finally, in the third-step cell search process, the UE detects a scrambling code assigned to a corresponding Node B among the 8 scrambling codes within the determined scrambling code group by searching CPICH after the frame synchronization is achieved. In order to perform the third-step cell search process in this manner, the UE must determine reference timing for where it will start the first-step cell search process, and reference timing for where it will start the second-step cell search process after acquiring time slot synchronization as the first-step cell search process is completed. That is, in order to identify a scrambling code group and acquire frame synchronization in the second-step cell search process, the UE must determine reference timing where it will perform cyclic shift so as to determine a frame boundary from the number of cyclic shifts performed during codeword decoding. In addition, after acquiring scrambling code group information and frame synchronization information through the second-step cell search process, the UE must determine reference timing for where it will start the third-step cell search process.
However, the UE must continuously perform such a cell search process for communication with Node Bs instead of performing the search only once at the initial cell search. Therefore, in order to calculate and manage a timing difference between a frame boundary of a signal currently demodulated by a UE and a frame boundary of signals received from the same Node B or different Node Bs, that may probably be demodulated later, a specified criterion is required. That is, it must be possible to calculate a timing difference between a frame boundary of neighboring Node Bs, detected through a cell search process, and a frame boundary of other Node Bs, detected by the previous cell search. In the multipath search, a UE calculates a correlation for scrambling codes of Node Bs to which the UE may probably move, and measures the strength of signals received from the Node Bs. However, since the Node Bs have different frame boundaries, the UE must initialize scrambling codes to match phases of the scrambling codes of the Node Bs before calculating the correlation. Therefore, it is necessary to provide reference timing for initializing the scrambling codes of the Node Bs. Multipath signal components determined to be valid by the cell search and the multipath search are assigned to a modulator or finger of a UE, for demodulation. The demodulator of the UE must initialize a scrambling code of a corresponding Node B synchronized with a frame boundary of a corresponding multipath signal, and provide reference timing for initializing the scrambling code. A device for proving the reference timing is a reference counter, and the reference counter continuously performs a count operation at periods of 10 ms by managing timing by the time slot. The count unit is 1/n chips, where n is the number of oversamples.
A mask operation or a slew operation is used to currently perform search on hypotheses to be searched when performing the third-step cell search or the multipath search. However, since the W-CDMA mobile communication system is an asynchronous mobile communication system, a UE has difficulty in performing a cell search on a plurality of Node Bs while in motion, and information that the UE holds for the cell search is limited. Particularly, the UE cannot store all mask values chip by chip (or in a chip unit) within 10 ms which is a unit of managing the reference timing. In addition, when a UE is designed to usually start the third-step cell search or the multistep search at a frame boundary, the UE should always wait for a frame boundary, suffering a loss in terms of demodulation timing, i.e., search rate. That is, the multipath search increases demodulation performance by demodulating and combining signals received through multiple paths, and when the UE usually starts the multipath search at a frame boundary, a delay occurs in actual combining due to the waiting time, causing a decrease in demodulation performance. Therefore, the UE suffers a loss in terms of demodulation timing.
Therefore, in general, a central processing unit (CPU) of the UE detects current timing by reading a current index value of the reference counter, determines an actual point of time where a searcher will operate, based on the detected current timing, and then sets a mask value for a phase of a corresponding scrambling code and activates the searcher.
As described above, in the third-step cell search and the multipath search, the CPU plays a part in determining an operation point of a searcher, and activates the searcher at the determined point of time. Of course, if the CPU fixes an operation point to a specific point instead of variably determining the operation point according to circumstances, the searcher will be usually activated at the fixed specific point of time. However, when cell search is performed in a channel environment, the search rate serves as an important parameter in the signal demodulation performance. In this case, the search rate must be high so that the UE can adaptively cope with a variation in the radio channel environment. However, the search rate is determined according to a plurality of parameters, such as hardware complexity, CPU speed, and software task scheduling. Therefore, in the case where there exists N search targets, in order to reduce a software load, it is possible to write by hardware all parameters related to the N search targets at once, thereby enabling a corresponding hardware to sequentially search the N search targets. The operation of searching by hardware for a plurality of search targets in a lump will be referred to as a “multisearch operation.”
In the W-CDMA mobile communication system, a counter operating in a frame unit (=10 ms), i.e., a reference counter, is used to continuously calculate a timing difference between Node Bs, i.e., a difference between frame boundaries. Herein, the reference counter will be referred to as an “index counter,” since an output of the reference counter is used as reference timing for a cell searcher, a multipath searcher and a multipath signal demodulator.
The index counter continuously operates, after being initialized by a CPU upon powering a UE. If a resolution of a searcher is 1/K chip, the index counter should also have a resolution of over 1/K chip. In the 3GPP specification, a length L of one frame is L=38,400 chips, the number M of time slots constituting one frame is M=15, and a length N of one time slot is N=2,560 chips. The length L of one frame becomes a period of one frame, and the length N of one time slot becomes a period of one time slot. A resolution of the searcher represents accuracy of the search, and is used to search for a more accurate synchronization point. Therefore, the minimum unit is 1 chip. In order to detect an accurate synchronization point, it is preferable to check a plurality of hypotheses or sampling points per chip and select an optimal point among the points.
FIGS. 2 and 3 illustrate different examples of a general index counter.
Referring first to FIG. 2, an index counter 210 comprises a slot counter 212 for counting time slots constituting one frame and a lower counter 214 for counting the number, K×N−1, of chips corresponding to a length of a predetermined number of time slots. A count value by the slot counter 212 ranges from 0 to M−1, and a count value by the lower counter 214 ranges from 0 to K×N−1. As stated above, in the W-CDMA mobile communication system, one frame comprises 15 time slots. The M, in this case, becomes 15. Further, in the W-CDMA mobile communication system, one time slot comprises 2,560 chips. The N, in this case, becomes 2,560. The lower counter 214 is reset to 0, when its count value becomes K×N. The slot counter 212 increases its count value by 1, each time the lower counter 214 counts a multiple (K×N) of N, i.e., counts the number N of chips constituting one time slot. The slot counter 212 is reset to 0, when the number of time slots constituting one frame is counted, i.e., the count value becomes M.
Referring now to FIG. 3, an index counter 310 has a structure for deriving count values of a slot counter 314 and a lower counter 316, using a single counter. The slot count 314 and the lower count 316 of FIG. 3 are substantially identical in function to the slot counter 212 and the lower counter 214 of FIG. 2, although they are represented by different reference numerals. A counter 312 constituting the index counter 310 counts from 0 to J×K×L−1, where J is an integer (J=1, 2, 3, . . . ). Count values of the slot counter 314 and the lower counter 316 are derived from a value counted by the index counter 310 in accordance with Equation (1).Slot count value=└(count value)/(K×N)┘ modulo M Lower count value=(count value) modulo (K×N)  Equation (1)
In Equation (1), └x┘ represents a maximum integer smaller than a given value “x,” and “a modulo b” represents a remainder obtained by dividing “a” by “b.”
Since the index counters 210 and 310 of FIGS. 2 and 3, respectively, operate at periods of a frame length, frame boundary points of all asynchronous cells (or Node Bs) are mapped to either specific count values of the slot counter 212 and the lower counter 214 in the index counter 210, or specific count values of the slot counter 314 and the lower counter 316 in the index counter 310. When the frame boundary point of the index counter 210 or 310 is defined as a reference point, a position of a frame boundary point of each asynchronous cell is called a “frame timing index” of the corresponding cell. Therefore, if a frame timing index of each cell is given, it is possible to calculate an offset between the cells. Here, the term “offset” means a difference between frame boundary points of the asynchronous cells.
In addition, it is possible to count a time slot length with the lower counter 214 or 316 of the index counters 210 and 310, respectively. Therefore, slot boundary points of all asynchronous cells can be mapped to a specific count value of the lower counter 214 or 316 in the index counters 210 and 310. When a slot boundary point of the lower counter 214 or 316 is defined as a reference point, a position of a slot boundary point of each asynchronous cell is called a “slot timing index” of the corresponding cell. Therefore, if a slot timing index of each cell is given, it is possible to calculate an offset between slot boundary points of the cells.
FIG. 4 illustrates an example of a timing relationship between an index counter and asynchronous cells. Specifically, FIG. 4 illustrates a timing relationship between two asynchronous cells Cell_A and Cell_B, by way of example. Referring to FIG. 4, an index counter's frame boundary 410, which is a reference point for determining a frame-boundary of the cells, may correspond to a count start point of the index counters 210 and 310. The slot counter 212 or 314 of the index counters 210 and 310 counts index counter's slot boundaries 412 up to M−1 after being reset to 0 at the index counter's frame boundary 410. The index counter's slot boundary 412, which is a reference point for determining slot boundaries in one frame, may correspond to a count start point of the lower counter 214 or 316 of the index counters 210 and 310. The lower counter 214 or 316 restarts counting, after being reset to 0 at the index counter's slot boundary 412.
A Cell A's slot boundary 426 is a point where the first-step cell search process is completed after the first-step cell search process is performed at a given time point T1=0. “T1=0” means that a count value of the lower counter 214 or 316 is 0. In this case, the first-step cell search process is performed at any one of the index counter's slot boundaries 412. If the Cell A's slot boundary 426 is determined, a Cell A's slot timing index 430 is determined by an offset between the Cell A's slot boundary 426 and the give time T1=0. For example, the Cell A's slot timing index 430 is determined by a count value of the lower counter 214 or 316 at the Cell A's slot boundary 426.
A Cell B's slot boundary 424 and a Cell B's slot timing index 428 are determined in the same process as used for the Cell A's slot boundary 426 and the Cell A's slot timing index 430. The Cell A's slot boundary 426 and the Cell B's slot boundary 424 correspond to one of count values 0 to NK−1, counted by the lower counter 214 or 316. An offset between the Cell A's slot boundary 426 and the Cell B's slot boundary 424 is defined as a slot offset 432. A Cell A's frame boundary 414 is determined by the number of cyclic shifts of codewords for a code group having a maximum correlation energy by performing the second-step cell search process at the Cell A's timing index 430. That is, a point spaced apart from the start point or the Cell A's slot timing index 430 of the second-step cell search process by a length of as many slots as the number x of cyclic shifts becomes the Cell A's frame boundary 414. A Cell A's frame timing index 418 is defined as a value counted by the slot counter 212 and the lower counter 214, or the slot counter 314 and the lower counter 316 at the Cell A's frame boundary 414.
A Cell B's frame boundary 416 and a Cell B's frame timing index 420 are determined using the same process that was used for the Cell A's frame boundary 414 and the Cell A's frame timing index 418. The Cell A's frame boundary 414 and the Cell B's frame boundary 416 are determined by one of count values 0 to M−1, counted by the slot counter 212 or 314, and the Cell A's slot boundary 426 or the Cell B's slot boundary 424. An offset between the Cell A's frame boundary 414 and the Cell B's frame boundary 416 is defined as a frame offset 422.
FIG. 5 illustrates an example of a cell search apparatus for a UE in a general W-CDMA mobile communication system. Referring to FIG. 5, a controller 510 controls an overall operation for cell search. Here, the controller 510 is identical in operation to the above-stated CPU. An index counter 514, having the structure illustrated in FIGS. 2 and 3, performs a counting operation stated above. A memory 512 stores a value counted by the index counter 514 in response to a save command from the exterior. The save command is divided into a lower count save command from a first-step searcher 518, a slot count save command from a second-step searcher 522, and a count save command from the controller 510. The memory 512 provides the controller 510 with the stored count value in response to the count save command from the exterior.
A first-step operation signal generator 516 generates a first-step search operation signal based on a count value provided from the index counter 514 in response to a first-step search command received from the controller 510. A first-step searcher 518 is initialized in response to an initialization command from the controller 510, and performs a first-step search operation on a received signal in response to the first-step search operation signal from the first-step operation signal generator 516. After completing the first-step search operation, the first-step searcher 518 provides the search result by the first-step search operation to the controller 510.
A second-step operation signal generator 520 generates a second-step search operation signal based on the count value provided from the index counter 514 in response to a second-step search command and a slot timing index by the first-step search operation, received from the controller 510. A second-step searcher 522 is initialized in response to an initialization command from the controller 510, and performs a second-step search operation on the received signal in response to the second-step search operation signal from the second-step operation signal generator 520. The received signal indicates a multipath signal, which is determined as a valid signal by the first-step search. After completing the second-step search operation, the second-step searcher 522 provides the search result by the second-step search operation to the controller 510. Meanwhile, the second-step searcher 522 commands the memory 512 to store a slot count value counted by the index counter 514, at a specific time point during the second-step search operation. For example, the second-step searcher 522 commands the memory 512 to store the slot count value counted by the index counter 514, at a point where the second-step search operation is started or a point where the second-step search operation is completed.
A third-step operation signal generator 524 generates a third-step search operation signal based on the count value provided from the index counter 514 in response to a third-step search command and the search result by the second-step search operation or a third-step search start point by a hypothesis for the second-step search result, received from the controller 510. A third-step searcher 526 is initialized in response to an initialization command from the controller 510, and performs a third-step search operation on the received signal in response to the third-step search operation signal from the third-step operation signal generator 524. The received signal means a multipath signal which is determined as a valid signal. After completing the third-step search operation, the third-step searcher 526 provides the search result by the third-step search operation to the controller 510. The controller 510 can determine a scrambling code for a target cell to be searched based on the search result from the third-step searcher 526.
A multipath operation signal generator 528 generates a multipath search operation signal based on the count value provided from the index counter 514 in response to a multipath search command and a multipath search start point, received from the controller 510. A multipath searcher 530 is initialized in response to an initialization command from the controller 510, and performs a multipath search operation on the received signal in response to the multipath search operation signal from the multipath operational signal generator 528. The received signal indicates a multipath signal, which is determined as a valid signal. After completing the multipath search operation, the multipath searcher 530 provides the search result by the multipath search operation to the controller 510.
A plurality of demodulation finger operation signal generators 532, 536 and 540 each generate a demodulation finger operation signal based on the count value provided from the index counter 514 in response to a finger demodulation command and a demodulation start point, received from the controller 510. The demodulation start point is a frame timing index indicating a frame boundary based on a scrambling code of a corresponding multipath signal to be demodulated. A plurality of demodulation fingers 534, 538 and 542 each are initialized in response to a corresponding initialization command from the controller 510, and perform demodulation on the received signal in response to the demodulation finger operation signals from the demodulation finger operation signal generators 532, 536 and 540. The received signal indicates a valid multipath signal, and the scrambling operation includes an initialization operation on the scrambling code.
However, the above-stated general cell searcher sequentially searches all search targets from the first search target to the last search target in the first-step cell search process, second-step cell search process, third-step cell search process and multipath search process, instead of multi-searching all the search targets in a lump. That is, a CPU writes search parameters for the first search target in the cell searcher. Thereafter, the cell searcher informs the CPU of a completed search on the first search target. The CPU then reads the completed search result provided by the cell searcher. Thereafter, as described in conjunction with the first search target, even for the next search target, the CPU writes search parameters for the corresponding search target in the cell searcher. Thereafter, the cell searcher informs the CPU of a completed search on the corresponding search target. The CPU then reads the completed search result by the cell searcher. In this manner, the CPU repeatedly performs a write operation for cell search and/or a read operation for reading search results, each time a search on each of all the search targets is completed. However, since the CPU performs a plurality of software tasks while controlling the overall operation of a UE, the repetition of the write and read operations for a cell search serves as a heavy load in operation of the CPU.